Polyurethanes are often used to cover rolls used in a variety of industries, e.g., paper, lumber, printing, steel, mining and textile industries. In particular, polyurethanes are often used when special properties are desired of the covered rolls, such as abrasion resistance, tear resistance, high load bearing with high hardness, and solvent resistance.
In the papermaking industry in particular, a plurality of rolls are used to transport a web or the paper from the beginning of the process through the end. In the process, a slurry of approximately 95% water and approximately 5% pulp fiber is transported via a web through machinery where water is extracted from the pulp, and the resultant pulp is then pressed and dried. In the process, a continuous sheet of paper is produced and wrapped onto a large metal roll for further processing. The rolls used in the papermaking process typically range in size from approximately 12 inches in diameter and a 100 inches long to approximately 60 inches in diameter and approximately 400 inches long. Fifty (50) to 250 rolls may be used in any one paper machine including the rolls at the end that the paper is wrapped upon, i.e., the reel spool. The outer diameter of these rolls is often covered with a rubber or polyurethane to protect the roll from corrosion and to help de-water the paper. They also provide traction for the webs used to transport the fiber and water sheet that is made into paper.
Polyurethane-covered paper machine rolls are known to have excellent abrasion resistance and corrosion resistance, vibration dampening and load-bearing ability. Further, polyurethane covered rolls help protect the web. The beginning of the papermaking machine 10, as depicted in FIG. 1, will typically carry the pulp on a web 12 (or “wire”) and will typically form a continuous loop that may encompass from approximately five to approximately thirty rolls and can be from approximately 100 inches to approximately 400 inches wide. On the wet end of the papermaking machine 10, the wire 12 may be exposed to temperatures of approximately 120 to 180 degrees Fahrenheit (° F.).
The wire 12 is usually a polypropylene screen that spins in a continuous loop along the rolls. Typically, only one or two rolls are driven. The driven rolls drive the wire and the wire drives the other rollers. The wire 12 is consumable and may cost from approximately $60,000 to approximately $100,000 each and usually lasts only approximately two to nine months. In the press section of the process, depicted in FIG. 2, the wire 24 is usually referred to as a “felt.” The polyurethane covering on rolls contacts this expensive consumable wire or felt at a pressure of approximately 15 to 60 pounds per linear inch to give the desired traction and performance.
Ideally, a roll and wire 12 track like a gear. If the wire 12 cannot keep up with the driven roll, then slippage occurs and the wire 12 is abraded and becomes worn. Anything that extends the life of the wire 12 or the felt 24 is considered a significant improvement in the process.
On the wet end of the papermaking machine 10, pulp is dispensed out onto the web 12 by a dispensing mechanism 14. The web 12 then travels across a series of foils 16 that de-water the pulp. Suction can be applied via the foils 16, or optionally, the foils 16 may have sharp edges that the pulp passes over that scrape the water off the bottom of the porous wire and creates a bit of vacuum on the trailing side of the pulp. After the pulp passes over foils 16, or optionally during passage over foils 16, a sheet of paper begins to form. The sheet disposed on web 12 then passes over vacuum boxes 18, and optionally beneath a dandy roll 19. After the sheet passes over the vacuum boxes 18, traditionally the sheet continues on to the press section 20, while the web 12 runs between at least one couch roll 21 and a suction pickup roll 22, and returns via guide rolls 23 to the beginning of the papermaking machine 10. Configurations vary by grade of product manufactured, but the process is essentially the same in all.
In the middle or press section 20 of the papermaking machine 10, as depicted in FIG. 2, the web is pressed between large press rolls 26 that may or may not apply suction through holes in the face of one of the press rolls. The cover on the rolls 22 helps determine the width of the nip and the pressure on the web 12. The sheet of paper, when picked up by the suction pickup roll 22, is pulled off of web 12 and onto the felt 24. The felt 24 comes around through the press and then is squeezed. The paper at this point is strong enough to accept a nip. In the press section 20, the paper and felt 24 may get nipped two or three times, and be transferred from one felt section to another felt section. As shown in FIG. 2, for example, the sheet of paper may pass through three felt sections.
On the dryer section 30 of the papermaking machine, as shown in FIG. 3, the web 12 is usually passed over steam-heated cylinders 32 that may be of a 350° F. internal steam temperature. In the dryer section 30, the sheet of paper is usually strong enough to be separated from the web 12 and travel on its own strength for short distances. While the paper may still be approximately 65% water, the fibers are usually bonded together enough from pressing and de-watering to form a sheet. In the dryer section 30, the paper travels back and forth through a series of steam-heated dryers 32. A felt may be used in the dryer section 30 to hold the paper down against the dryer cylinder 32 and to further absorb the water that is coming out of the paper.
At the very end of the papermaking machine 10 in section 40, as shown in FIG. 4, the paper may go through a calender reel 42, to which various coatings, sizings, etc. may be placed on the paper, e.g., for printability. The paper is then wound up on a reel spool 44.
The typical process for applying polyurethane to the rolls described hereinbefore is a vertical casting process where the roller is picked up on one end and lowered into a mold that is customized for the roller. The mold is then poured, casting the bigger outer diameter on the outside of the roll. Post curing typically takes place in the mold. The roll is then removed from the casting and then tooled or ground to get the evenness and finish that is desired on the surface of the polyurethane.
There are some problems associated with the vertical casting process; for example, the polyurethane may disbond from the roll. Further, bubbles may be formed in the polyurethane which it is necessary to remove. An additional problem with conventional casting processes is that the custom molds are built for each roll and the polyurethane is cast with a large amount of extra stock on them because of surface defects that occur in the mold due to gas bubbles and the abuse to which the mold is subjected. This is an extremely time-consuming and expensive process and requires a lot of storage space for the molds. Further, the conventional casting process is probably not necessary for 85% of rolls in paper-making process that do not need to withstand high temperatures and/or high pressure.
Another process used for applying polyurethane to the roll is a rotational casting process, which is performed horizontally. In the rotational casting process, polyurethane is ribbon-flowed onto the surface of a shell as it rotates. The head of a polyurethane-dispensing mechanism traverses the roll, extruding polyurethane onto the surface of the roll at a very low pressure and flow rate. Because the polyurethane is a liquid when dispensed, the liquid must be slowly extruded so that it does not drip off of the roll during the dispensing process. Further, the polyurethane in the traditional rotational casting process is only applied in a four-inch width, and under a pressure of approximately 1,000 pounds per square inch (psi) or less.
Additionally, traditional rotational casting processes require that the roll be placed in an oven for curing, as well as additional post-cure cooling time before machine grinding the roll. The traditional rotational casting process can take up to approximately 16 hours from beginning until the roll is removed from the oven. Adding in post-curing time, it can take up to approximately 24 hours to completely cover one roll before machining or grinding can begin.
The polyurethanes that are processed in the rotational casting process are typically made from polyethers. One problem with the polyether chemicals suitable for this process is that they cannot be used in the wet end of the paper machine (FIG. 1) because they absorb too much water. Therefore, they are usually used only on reel spools 44 at the very end section 40 of the machine 10 (FIG. 4). This is a more limited market, and is also a very expensive technology.
Rubber has also been used to cover and protect the rolls and is the oldest technology that has been used to cover rolls. The rubber is typically applied in an extrusion process that extrudes a ribbon on the roll starting at one end and traversing to the other, as the roll rotates. A lot of excess rubber is usually applied because the rubber has to be vulcanized on the entire roll and it shrinks in this process. An additional problem with the use of rubber is the requirement of an oven that is big enough to accommodate the entire roll core. The rubber is then cooked until it is cured on the core, resulting in a shrinkage that stresses the cover. The rubber then has to be rough tooled and ground to get the straightness and finish that is needed on the surface of the cover. Because the process of applying rubber is very inaccurate, a large amount of wasted material is usually machined off to create the desired surface. For example, in a typical process of preparing a rubber-covered roll, approximately 0.250 inch per side (0.5 inch on diameter) to approximately 0.5 inch per side (1.0 inch on diameter) is machined off. The process of applying rubber to the typical papermaking roll may take approximately three to four days, which renders this a time consuming process.
In the rubber extrusion process, for example, a ribbon of rubber is extruded that may be one-half inch (½ in.) wide and approximately one quarter (¼) to three-eighths (⅜) of an inch thick. That is sometimes accomplished at an angle, or vertically with the narrow end against the roll. The result is a very rough surface as the rubber is extruded because each ribbon is pressed against another ribbon, as they are stacked in multiple layers. Because the layers of rubber are being pressed, the top layer tends to bunch up more in the area of the joints. This can lead to as much as one-quarter of an inch (¼ in.) difference in the surface between the low spot and the high spot between the center of one ribbon and the joint with the next ribbon. The result is that this portion of the rubber must be machined off once it is cured. Therefore, a lot of stock must be removed in order to obtain a 100% clean rubber surface, which results in a large amount of wasted material.
Polyurethane has also been dispensed on to various objects, from truck beds to roll covers, through a spraying mechanism. The problem with the typical spraying process, however, is that the polyurethane is typically applied in only approximately {fraction (40/1000)} of an inch in a pass, and therefore a three-quarter inch (¾ in.) cover would require approximately 20-40 passes about the roll in order to build up the thickness of the polyurethane.
Thus, a heretofore unaddressed need exists in the industry to address the aforementioned deficiencies and inadequacies.